Manufacturers of internal combustion engines develop engine operation control strategies to satisfy customer demands and meet various regulations for emission control and fuel economy. One such engine control strategy comprises operating an engine at an air/fuel ratio that is lean of stoichiometry to improve fuel economy and reduce greenhouse gas emissions. Such operation is possible using both compression-ignition (diesel) and spark-ignition engines. When an engine operates with a lean (excess oxygen) air/fuel ratio, the resultant combustion temperature and excess oxygen leads to higher engine-out NOX; however, commercial application of lean-operating engines is limited due to lack of effective methods to remove NOX from an exhaust gas flow under lean exhaust conditions. Thus, efficient reduction of nitrogen oxides (NOX═NO+NO2) from lean-burn diesel and gasoline engine exhaust is important to meet future emission standards and improve vehicle fuel economy.
Reduction of NOX emissions from an exhaust feedstream containing excess oxygen is a challenge for vehicle manufacturers. By way of example, it is estimated that compliance with Bin 5 regulations in the United States may require an aftertreatment system capable of 70-90% NOX conversion efficiency on the FTP (Federal Test Procedure) cycle based on currently anticipated engine-out NOX levels. For practical application, the conversion efficiency must be obtained over a range of relatively low operating temperatures (e.g., 200-350° C.) occurring during the aforementioned FTP cycle and at a relatively higher range of operating temperatures (e.g., 450-550° C.) occurring during a high speed test cycle (e.g., US06 federal test procedure).
Various exhaust gas treatment systems have been proposed for vehicle applications under these lean-burn operating conditions. One approach combines a NOX storage reduction catalyst in series with a downstream diesel particulate NOX reduction system and a further downstream diesel oxidation catalyst. Targeted performance of 75-85% NOX reduction has been reported for such systems. However, such systems require periodic catalyst regeneration involving fuel injection to generate high exhaust gas temperatures and injection of reductants to regenerate the storage material of the catalyst. During periods of catalyst regeneration, maintaining the targeted NOX reduction levels during operation of the vehicle becomes problematic, since the components being regenerated will have greatly reduced NOX conversion capability.
Accordingly, there remains a need for effective exhaust gas treatment systems and methods of using the same to selectively reduce NOX in the exhaust gas flows of lean-burn internal combustion engines, particularly those used in various vehicular applications.